1. Field of the Invention
This invention relates to cerium-magnesium aluminate luminescent materials. More particularly it relates to ultraviolet emitting luminescent materials when excited by 254 nanometer energy, processes for making the materials and lamps utilizing these materials.
2. Prior Art
Verstegen et al in J. Luminescent 6 425 (1973) and in Belgian Pat. No. 7,214,862 disclose the compound, CeMgAl.sub.11 O.sub.19, having the magnetoplumbite structure. The above Belgian patent discloses the materials in the triangle labeled ABC in FIG. 1. The materials within that area can be described by the formula: EQU Ce.sub.x Mg.sub.y Al.sub.11 O.sub.16.5 + 1.5x + z
wherein x is from about 0.95 to about 5.5 and y is from 0 to about 5.5 or alternatively expressed in terms of molar percentages as the cerium oxide content (on a CeO.sub.1.5 basis) varies from about 8 to about 33 mole percent, the magnesium oxide (on a MgO basis) varies from about 0 to 33 mole percent and the aluminum oxide content (on a AlO.sub.1.5 basis) varies from about 67 to about 92 mole percent.
The compound, CeMgAl.sub.11 O.sub.19, has a peak emission at about 373 nanometers when subjected to 254 nanometer excitation. The materials containing the relatively high percentages of CeO.sub.2 and MgO in the system lose brightness quite rapidly at elevated temperatures. For example, the intensity or brightness of CeMgAl.sub.11 O.sub.19 (about 7.7 mole percent CeO.sub.1.5 and about 7.7 mole percent MgO) steadily drops above about 125.degree. C and at 400.degree. C is only about 35% of its maximum intensity or brightness.
It is believed that a new family of materials in the CeO.sub.2 . MgO . Al.sub.2 O.sub.3 system having an improved temperature dependence and in which up to 50 mole percent of the CeO.sub.2 can be replaced with an alkaline earth oxide, RO, wherein R is selected from Sr, Ca and Ba, would be an advancement in the art. It is also believed that lamps utilizing the above-mentioned luminescent materials which can be operated at wall temperatures up to the 300.degree. C to 400.degree. C range would be an advancement in the art.